The key here is that you have to make sure you're studying what the GMAT tests - that's not necessarily the same as what another grammar source says is acceptable. Wren & Martin is a very reputable source. But the GMAT tends to cling to the old formal rules much longer, while more modern sources will adapt more quickly to recent changes.

Because the GMAT people are writing the test, we have to go by what they say, not by what modern usage may allow in everyday speech or writing.

I think Wren & Martin (or something similar) can be a good secondary resource if you want some additional information about something you read, but use GMAT-based works (such as OG or our SC guide) as your primary source and, if there are contradictions, go with what the GMAT-based source says.